The present invention relates to apparatus for fluidizing a particle bed during gas coating and more particularly to such apparatus where the fluidized bed is developed by the introduction of levitating gas flow.
It is well known in the prior art to employ gas coating within a fluidized bed for coating particles and other objects immersed in the bed. The coating may be formed by the decomposition, polymerization, condensation and/or chemical reaction of a gas, vapor or gas-vapor mixture. As a particular example of such a process, hydrocarbon products are commonly applied in such a coating process to provide protection for nuclear fuel particles of a type used in nuclear reactors. The fuel particles are small, for example, on the order of 500 microns and may be formed from a suitable fissile and/or fertile material, such as uranium, plutonium, thorium or suitable compounds of such materials.
Within a nuclear reactor, the fuel particles are exposed to conditions of high temperature and severe irradiation over prolonged periods of operation. In order to assure continued effectiveness under these conditions, it has become common practice to coat the fuel particles with an impermeable material in order to retain gaseous and metallic fission products within the individual particles.
Pyrolytic carbon and metallic carbide are specific examples of materials composing such coatings for nuclear fuel particles. The coatings may be applied at high temperature through the introduction of a reactant gas including or consisting of a suitable hydrocarbon such as acetylene, propylene, propane or methane. The desired coating is deposited through the high temperature decomposition of the hydrocarbon gas.
Examples of fuel particles provided with such coatings are disclosed and set forth for example in U.S. Pat. No. 3,325,363 issued June 13, 1967 to Goeddel et al.; U.S. Pat. No. 3,298,921 issued Jan. 17, 1968 to Bokros et al.; U.S. Pat. No. 3,361,638 issued Jan. 2, 1968 to Bokros et al.; and U.S. Pat. No. 3,649,452 issued Mar. 14, 1972 to Chin et al.
When the particles being coated are relatively small, the coating operation may be efficiently carried out with the particles suspended in the form of a fluidized bed within a high temperature coating chamber. Levitation of the particles within the fluidized bed may be achieved through the controlled introduction of a hydrocarbon gas, an inert carrier gas or a combination thereof into the coating chamber for dispersion through the particle bed. Most commonly, an inert carrier gas such as argon, helium, nitrogen or hydrogen is employed for this purpose.
In a coating chamber for carrying out such a coating operation, the particles are dispersed and suspended by means of the levitating gas in order to form the fluidized bed. A reactant gas is introduced by a nozzle and decomposed at high temperatures within the coating chamber in order to deposit the coating material upon the particles. Various conditions for carrying out such a coating operation are believed to be well known in the prior art including temperature ranges within the coating chamber as well as rates and pressures under which both the reactant and levitating gases may be introduced into the chamber as well as the duration of the coating operation.
High temperature gas coating operations employing fluidized beds of particles are relatively complex and may accordingly involve numerous problem areas. In particular, it has been found that a substantial deposition or buildup of carbonaceous material occurs upon internal surfaces of the chamber during decomposition of the reactant gas within the high temperature environment. Carbon buildup is a particular problem when it tends to disrupt the proper introduction of reactant or levitating gases or to interfere with the circulation of particles throughout the bed or the removal of coated particles from the chamber. When the reactant gas is introduced into the coating chamber by means of a nozzle extending upwardly from a central portion of the coating chamber base, severe deposition tends to develop upon the floor of the coating chamber. Carbon buildup in this location particularly interferes with the unloading of the coated particles from the chamber by means of an opening in the chamber floor.
An additional problem area of particular note in relation to the present invention involves the need for maintaining uniform distribution and circulation of particles throughout the fluidized bed. These factors are especially important in developing uniform coating of entire particle batches in the coating chamber.
Accordingly, there has been found to remain a substantial need for improved apparatus for dispersing and suspending particles in a fluidized bed during gas coating.